Monolayer MoS2 Fabricated by In Situ Construction of Interlayer Electrostatic Repulsion Enables Ultrafast Ion Transport in Lithium-Ion Batteries

Highlights In-situ construction of electrostatic repulsion between MoS2 interlayers is first proposed to successfully prepare Co-doped monolayer MoS2 under high vapor pressure. The doped Co atoms radically decrease bandgap and lithium ion diffusion energy barrier of monolayer MoS2 and can be transformed into ultrasmall Co nanoparticles (~2 nm) to induce strong surface-capacitance effect during conversion reaction. The Co doped monolayer MoS2 shows ultrafast ion transport capability along with ultrahigh capacity and outstanding cycling stability as lithium-ion-battery anodes. Abstract High theoretical capacity and unique layered structures make MoS2 a promising lithium-ion battery anode material. However, the anisotropic ion transport in layered structures and the poor intrinsic conductivity of MoS2 lead to unacceptable ion transport capability. Here, we propose in-situ construction of interlayer electrostatic repulsion caused by Co2+ substituting Mo4+ between MoS2 layers, which can break the limitation of interlayer van der Waals forces to fabricate monolayer MoS2, thus establishing isotropic ion transport paths. Simultaneously, the doped Co atoms change the electronic structure of monolayer MoS2, thus improving its intrinsic conductivity. Importantly, the doped Co atoms can be converted into Co nanoparticles to create a space charge region to accelerate ion transport. Hence, the Co-doped monolayer MoS2 shows ultrafast lithium ion transport capability in half/full cells. This work presents a novel route for the preparation of monolayer MoS2 and demonstrates its potential for application in fast-charging lithium-ion batteries. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01042-4.


Fig. S2 (a-c) TEM images of MoS2
The doping amount of N and O elements in the carbon materials should be calculated by N (at%)/(C (at%)+N (at%)+O (at%))*100% and O (at%)/(C (at%)+N (at%)+O (at%))*100%, respectively. The doping amount of Co elements in the MoS2 materials should be calculated by Co (at%)/(Co (at%)+Mo (at%)+S (at%))*100%, respectively. The specific doping amount is represented in Table S2 and S3.    The C, N, O, and S contents in the composites were measured using O/N/H and C/S elemental analyzers. The Co contents in the composites were tested by inductively coupled plasma mass spectrometer. The content of Mo was calculated as a difference to 100 wt%. It can be seen that the mass percentages of N, O codoped carbon matrix are 18.1, 19.2, 20.4, and 21.7wt%, corresponding to MoS2/C, CoMoS2/C-I, CoMoS2/C-II, and CoMoS2/C-III, respectively.
After heating in air atmosphere, the increase of the mass is ascribed to the oxidation of Mo and Co into MoO3 and Co2O3, while the decrease of the mass is attributed to the oxidation of C, N into CO2 and NO2 and the mass loss of O. Consequently, the final residual products are MoO3 and/or Co2O3.
According to the element analysis results (Table S4), the mass of the final residual products for pure MoS2, MoS2/C, CoMoS2/C-I, CoMoS2/C-II, and CoMoS2/C-III should be 88.9, 73.4, 69.6, 66.2, and 60.1wt%, respectively.  (d); (e, f) CoMoS2/C-I before cycling (e) and after 100 cycling (f); (g, h) CoMoS2/C-II before cycling (g) and after 100 cycling (h); (i, j) CoMoS2/C-III before cycling (i) and after 100 cycling (j)  The larger slope of inclined line in the low frequency represents lower ion diffusion impedance.  Higher specific surface area can ensure more fully contact between active materials and electrolyte, thus boosting the lithium-ion transport. MoS2/C 4.6 CoMoS2/C-I 23.4 CoMoS2/C-II 56.3 CoMoS2/C-III 18.5 The higher EC is more favorable for charge transport.

EC measurements
First, the obtained powders were added in a hollow cylinder mould with two electrodes on each ends, which were connected with a digital multimeter (Keithley 2001, USA) and followed by compressing powders into slices. During compressing the electrical resistance was observed constantly. When the electrical resistance kept stable the mould was opened, obtaining slices. Subsequently, the EC of slices was measured using a four-probe tester (Probes Tech RTS-8, China).

Fig. S16 Nyquist plots of CoMoS2/C-II at different cycling number
Fig. S17 Cycling performance of CoMoS2/C-II at 2 A g -1 Nano-Micro Letters S10/S16     Fig. S27b, which should be ascribed to the conversion reaction of large-sized Co3S4. The smaller nanoparticles are obtained in Fig. S27c, which should be ascribed to the conversion reaction of Co-doped monolayer MoS2. The average size of Mo and Co nanoparticles is calculated based on the amount of particles in Fig. S27a, and the size of particles in Figs. S27b, c, which is about 3.5 nm Table S10 Electrochemical performances of MoS2-based anode materials in LIBs full cell. CC-charge capacity (mAh g -1 ), CR-capacity retention (%), ML-mass loading (mg cm -2 ), J-current density (A g -1 , based on cathode), NC-cycle number, NA-not available.